Subcutaneous defibrillation electrodes

ABSTRACT

Implantable electrodes for defibrillation are formed of pluralities of electrode segments. Each of the segments is relatively long and narrow. The electrode segments can be parallel and spaced apart from one another a distance at least ten times the nominal width, with one end of each segment mounted to a transverse distal portion of an electrically conductive lead coupling the electrode to a defibrillation pulse generator. Alternatively, segments can branch or radiate outwardly from a common junction. In yet another arrangement, electrode segments are portions of a single conductive path at the distal end of a lead from a pulse generator, arranged in either a spiral configuration or a serpentine configuration which can align electrode segments side by side, parallel and spaced apart. The electrode segments can be formed of composite conductors in the form of titanium ribbons or wires with a sputtered outer layer of platinum, or a silver core in a stainless steel tube, with a platinum layer formed onto the tube. The electrodes are highly compliant yet can provide large effective areas for defibrillation, enabling a transthoracic pulsing arrangement of two electrodes on opposite sides of the heart, implanted subcutaneously outside of the thoracic region.

This is a continuation of copending application Ser. No. 07/533,886,filed on Jun. 6, 1990, now U.S. Pat. No. 5,203,348.

BACKGROUND OF THE INVENTION

The present invention relates to field of electrical defibrillation,including cardioversion, and more particularly to the structure for anelectrode used in implantable defibrillation systems The term"defibrillation" as used herein, includes cardioversion which is anothertechnique involving relatively high energy delivery, as compared topacing, as well as other aspects of defibrillation therapy such as themonitoring of cardiac electrical activity (sensing) when not deliveringhigh energy impulses.

Defibrillation is a technique employed to counter arrhythmic heartconditions including some tachycardias, flutter and fibrillation in theatria and/or ventricles. Typically, electrodes are employed to stimulatethe heart with electrical impulses or shocks, of a magnitudesubstantially greater than pulses used in cardiac pacing. Onedefibrillation approach involves placing electrically conductive paddleelectrodes against the chest of the patient. During cardiac surgery,such paddles can be placed directly against the heart to apply thenecessary electrical energy.

More recent defibrillation systems include body implantable electrodes.Such electrodes can be in the form of patches applied directly toepicardial tissue, or at the distal end regions of intravascularcatheters, inserted into a selected cardiac chamber. U.S. Pat. No.4,603,705 (Speicher et al), for example, discloses an intravascularcatheter with multiple electrodes, employed either alone or incombination with an epicardial patch electrode. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. No. 4,567,900(Moore).

Epicardial electrodes are considered the most efficient, in the sensethat less energy is required for defibrillation as compared to eitherchest contact paddles or intravascular catheter electrodes. Howeverepicardial electrode implantation is highly invasive, major surgery,since it is necessary to enter the chest cavity, which typicallyinvolves spreading of adjacent ribs or splitting of the sternum. Thisprocedure presents a risk of infection. Further, implantation andattachment place physical constraints upon the nature of electrode.These electrodes must be either quite small, or extremely compliant andresistant to fatigue, as they maintain conformal fit to the contractingheart.

Generally, larger defibrillation electrodes are considered moredesirable, since they reduce the impedance at or near the electrode.Sensing artifacts also are reduced for larger electrodes. However,larger electrodes are difficult to attach to the epicardium, as theymust conform to the heart during the contractions associated with normalcardiac activity. Subcutaneous electrodes are more easily implanted, atless risk to the patient. In a defibrillation electrode or any otherimplanted device, however, increasing the size generally increasesdiscomfort and surgical risk to the patient.

Increasing the size of a defibrillation electrode affects its electricalperformance. Conventional electrodes are subject to "edge effects"arising from the non-uniform distribution of electrical energy when theelectrode receives the pulse. In particular, current densities aregreater at the edges of the electrode than at interior regions Of theelectrode. An attempt to counter the edge effect is disclosed in U.S.Pat. No. 4,291,707 (Heilman et al). A series of circular openings,through an insulative layer framing a conductive screen are said tosubstantially eliminate the edge effect by the additional exposure ofthe screen. Another problem encountered in larger electrodes is theresistance across the length (largest linear dimension) of theelectrode, leading to unwanted voltage gradients across the electrodewhich can degrade electrode performance.

Therefore, it is an object of the present invention to provide animplantable defibrillation electrode with a large effective surface areato lower the impedance at or near the electrode, without causing unduepatient discomfort.

Another object is to provide a defibrillation electrode that has a largeeffective area, yet is easier to implant and readily conforms to thecontours of its implant location.

A further object is to provide a defibrillation electrode structureenabling a relatively large size while reducing the non-uniform fielddistribution associated with conventional electrodes.

Yet another object is to provide defibrillation electrodes of sufficientsize and effectiveness to enable transthoracic delivery ofdefibrillation pulses, with an implanted system.

SUMMARY OF THE INVENTION

To achieve these and other objects, there is provided a body implantabletissue stimulating electrode. The electrode includes a plurality offlexible, electrically conductive electrode segments having a nominalwidth and a length at least five times the nominal width. A means isprovided for mechanically coupling the electrode segments with respectto one another whereby each of the segments, over the majority of itslength, is spaced apart from each one of the other segments by adistance of at least 1.5 cm. A means is provided for electricallycoupling the electrode segments for substantially simultaneous receptionof the tissue stimulating electrical pulses from a pulse generatingmeans. Consequently the electrode segments, when receiving the tissuestimulating pulses, cooperate to define an effective electrode areaincorporating the electrode segments and having a width of at least 1.5cm.

In one preferred configuration, the electrode segments are linear and inparallel spaced apart relation, all extending in a longitudinaldirection. The mechanical and electrical coupling means can be atransversely extended distal portion of an elongate, electricallyconductive lead. The lead is connected to each of the respective firstend portions of the electrode segments along its distal region, andconnected at its proximal end to a pulse generating means. Preferably anelectrically insulative layer covers the lead, leaving the electrodesegments exposed, to define a substantially rectangular "phantom" areaor effective electrode area.

Alternatively, the electrode segments can radiate outwardly from acommon junction, typically at the distal end of the lead or conductivecoupling wire from the pulse generating means. While the coupling wireis covered with an insulative material over the majority of its length,a distal end portion of the coupling wire can be left exposed, toprovide one of the electrode segments.

Yet another approach involves a single electrically conductive wire orpath, with portions of the path providing the spaced apart segments. Asan example, the path can be arranged in a serpentine configuration inwhich segments are parallel to and aligned with one another, side byside. Alternatively, the conductive path is formed as a spiral. Ineither event, adjacent segments are spaced apart from one another adistance substantially greater than their width, preferably by an orderof magnitude or more.

In a preferred example, elongate electrode segments about 30 cm long andwith a nominal width of 0.5 mm extend longitudinally, aligned with oneanother and spaced apart from one another by about 3 cm. One end of eachelectrode segment is mounted to the distal end portion of a conductivelead to a pulse generator. At the opposite, free end of each segment isan enlargement such as a loop or flared end, formed to minimize localhigh current densities due to the previously described edge effects. Thecombination of a large phantom area with multiple conductive segmentsreduces non-uniform current distributions.

The best results are achieved with highly conductive electrode segments.Accordingly, the segments are preferably formed of low resistancecomposite conductors including drawn braised strands (DBS), drawn filledtubes (DFT) and the like, coated with platinum or another metal from theplatinum group, e.g. iridium, ruthenium or palladium, or alternativelywith an alloy of one of these metals. The strands can be formed oftitanium or platinum. A suitable filled tubular conductor is composed ofa silver core within a stainless steel tube. The electrode segments canbe formed of single wires, pluralities of wires in a braided or twistedconfiguration, helically wound coils, or a woven mesh or screen. In someembodiments, particularly those employing the woven screen, it isfurther desirable to include an insulative backing to more positivelyposition the electrode segments with respect to one another.

It has been found that highly conductive electrode segments reduce anyvoltage gradient across the electrode, with the separate segmentssimultaneously receiving a defibrillation or other stimulation pulse.The separate segments thus cooperate to act as a single "patch"electrode, having an effective surface area equal to that of a rectangleor other polygon containing all of the segments. As an example, anelectrode formed as a row of five parallel electrode segments spacedapart from one another by 3 cm, each segment being 10 cm long, wouldhave a rectangular phantom or effective area slightly larger than 120(twelve times ten) square cm. Yet, as compared to a continuousrectangular patch electrode measuring ten by twelve cm, the branchedsegment electrode in accordance with the present invention is easier toimplant, reduces the high current density regions, and more easilyconforms to the thorax or other surface to which it is attached. Infact, branched arrangements of segments can provide effectivedefibrillation electrode areas in the range of from 100 to 200 squarecm, while enabling easy implantation.

Thus, in accordance with the present invention there is disclosed aprocess for applying defibrillation pulses to a human heart, includingthe following steps:

(a) implanting a first compliant electrode in a patient, proximate thepleural cavity and the rib cage, and on a first side of the thoracicregion of the body;

(b) implanting a second compliant electrode in the body, proximate thepleural cavity, and the rib cage, and on a second side of the thoracicregion opposite the first side, with at least a portion of the heartbetween the first and second electrodes;

(c) implanting a defibrillation pulse generator; and

(d) electrically coupling the first and second electrodes to adefibrillation pulse generator and providing defibrillation pulses fromthe pulse generator across the first and second electrodes.

If desired, one or more electrodes implanted proximate the pleuralcavity and rib cage can be used in combination with one or more coilelectrodes mounted on an intravascular catheter, preferably positionedin the right atrium and the right ventricle of the heart, with thedistal end of the catheter near the apex of the right ventricle.

As compared to the entry into the chest cavity normally associated withimplanting epicardial electrodes, transthoracic placement ofsubcutaneous electrodes as outlined above is substantially lessinvasive, preserves the integrity of the rib cage and the pleuralcavity, and reduces risk of infection.

Nonetheless, other implant locations, including direct attachment toepicardial tissue, can be employed in accordance with the presentinvention, to achieve relatively large effective electrode areas whilemaintaining patient comfort with substantially more uniform distributioncurrent density.

IN THE DRAWINGS

For a further understanding of the above and other features andadvantages, reference is made to the detailed description and to thedrawings, in which:

FIG. 1 is a top plan view of a defibrillation electrode constructed inaccordance with the present invention;

FIG. 2 is a sectional view taken along the line 2--2 in FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 in FIG. 1;

FIG. 4 is a top plan view of an alternative embodiment electrodeconstructed in accordance with the present invention;

FIGS. 5-9 illustrate alternative constructions for electrode segments ofthe electrodes;

FIG. 10 is plan view of another alternative embodiment electrodeconstructed in accordance with the present invention;

FIGS. 11-13 illustrate further alternative configurations of theelectrode of FIG. 9;

FIG. 14 is a top plan view of another alternative embodiment electrode;

FIGS. 15, 16 and 17 illustrate a further embodiment electrode;

FIG. 18 is a top plan view of yet another embodiment electrode;

FIG. 19 is a schematic representation of the electrical field between acontinuous patch electrode and an electrode having segments, but inwhich the segments are too close to one another;

FIG. 20 is a schematic representation of the electrical field betweentwo electrodes constructed according to the present invention;

FIG. 21 is a plot of intraelectrode impedance as a function of thespacing between adjacent segments of each of the electrodes, forelectrodes with from two to four segments; and

FIGS. 22, 23 and 24 diagrammatically illustrate alternative implantationapproaches for defibrillation systems incorporating electrodes embodyingthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, there is shown in FIG. 1 a defibrillationelectrode 16 including three parallel and spaced apart electrodesegments 18, 20 and 22. Each of the segments has a length (L in thefigure) substantially longer than its width (W), e.g. 30 cm long with anominal width preferably about 0.5 mm. Generally, the width should bewithin the range of from 0.25-5 mm. Adjacent segments are spaced apart adistance (D) substantially greater than the nominal width, e.g. 3 cm.This center-to-center spacing should be at least 1.5 cm, and preferablydoes not exceed 30 cm.

Electrode segments 18, 20 and 22 are fixed at respective first ends to adistal end portion 24 of an electrically conductive lead 26. The leadconducts electrical pulses to the electrode segments from a pulsegenerator (not shown) coupled to the proximal end of the lead. Lead 26at the distal end structurally supports the longitudinally extendedelectrode segments in the transversely spaced apart configuration shown.

The electrically conductive portion of lead 26 is surrounded by anelectrically insulative cover or sheath 28, preferably constructed of abody compatible polymer, e.g. a medical grade silicone rubber orpolyurethane. As seen in FIG. 2, the lead includes a composite conductorformed of a core 30 of silver surrounded by a tube 32 of stainlesssteel. This type of composite conductor is known as drawn field tube(DFT) of MP35N (brandname) alloy available from FWM Research Products ofFort Wayne, Ind. Further, a coating 34 of platinum is applied over thestainless steel, preferably by sputtering or other deposition process.While preferably platinum, coating 34 also can consist of another metalfrom the platinum group (e.g. iridium, ruthenium and palladium) or analloy of these metals. Insulative sheath 28 is contiguous with andsurrounds the platinum layer.

As seen in FIG. 3, the construction of electrode segment 22 (andlikewise segments 18 and 20) over substantially all of its length issubstantially similar to the construction of the conductive portion oflead 26. Thus the segments also are highly electrically conductive.,Platinum coating 34 provides a further advantage for the segments, whichare not covered by the insulative sheath. In particular, the platinumcoating when applied by vapor deposition provides a microtexture whichsubstantially increases the reactive surface area of the electrodesegments, to reduce near field impedance of the electrode (the term"near field" impedance refers to the voltage losses associated with theelectrode due to chemical and field effects). For a further discussionof this feature, reference is made to U.S. Pat. No. 5,074,313, andassigned to the assignee of the present application. The reducedinterface impedance increases the ratio of bulk impedance to the totalsystem impedance as measured between the stimulating electrode and theindifferent or signal return electrode. Thus, more of the voltage dropoccurs across tissue, where it is useful for causing the desiredstimulation, with proportionately less of the voltage drop occurring atthe electrodes where it is non-productive. This enables a reduction inoverall potential or pulse duration, in either event reducing therequired energy for defibrillation.

Given adequate separation between segments 20, 22 and 24, the currentdistribution is made more uniform. To further counter any currentdensity differentials due to edge effects at the ends of segments 20, 22and 24, loops 36, 38 and 40 are formed at these ends, respectively.Alternatively, the ends can be flared or otherwise enlarged, and remainsubstantially free of undesirable concentrations of high current. Suchenlargements also facilitate implant, as they tend to positionally fixthe electrode segments.

Because the electrode segments are electrically common, the electrodesreceive and transmit defibrillation pulses simultaneously. The electrodesegments are sufficiently near one another to function in concert,providing an effective area or phantom area incorporating the segments,as indicated in broken lines at 42. In other words, electrode segments20, 22 and 24 define a generally rectangular effective area, withsubstantially greater compliance to contours and movements of bodytissue, as compared to a continuous patch electrode. In addition, thespacing between electrodes performs an important electrical function byproducing a substantially more uniform current distribution than that ofa continuous patch electrode. Patch electrodes are known to have regionsof very high current density around their outside edges, and regions oflow current density at their centers. By using a segmented electrode,with segments properly spaced apart from one another, much highercurrents can be delivered to the central region of the effective orphantom area because current is able to flow between adjacent segments.This results in a more uniform electrical field across the heart.

FIG. 4 illustrates an alternative embodiment defibrillation electrode 44including five elongate electrode segments 46, 48, 50, 52 and 54, eachwith a preferred width and substantially greater preferred length asdescribed in connection with electrode 16. Each of electrode segments46-54 is part of a wire mesh pattern 55 and extends longitudinally.Transversely extended end portions 56 and 57 of the pattern couple thesegments to a lead 58. An insulative sheath 62 surrounds lead 58 fromelectrode 44 to the proximal end of the lead. An electrically insulativebacking 64 supports mesh pattern 55. The mesh pattern is covered by aninsulative layer 66. Slots 68, 70, 72 and 74 are formed in backing 64and layer 66 between adjacent electrode segments.

FIG. 5 illustrates an alternative form of composite conductor known asDBS drawn braised stranded, available from FWM Research Products, FortWayne, Ind. As shown, a silver core 73 is surrounded by six stainlesssteel wires 75. The structure is heated and drawn to braise all wirestogether. The results is a solid, continuous composite conductorcomposed of a silver core and a stainless steel outer shell or tube.

FIG. 6 illustrates an alternative construction for the electrodesegments of either electrode 16 or electrode 44, involving a pluralityof composite conductors 76 in a twisted configuration. Each of theconductors can include a silver core within a stainless steel tubecoated with platinum as previously described. Alternative compositeconductors for single and multiple wire arrangements include platinum ortitanium ribbon or wire, clad with platinum. The twisted constructionenhances flexibility and resistance to fatigue in the electrodesegments. Other alternatives include braided or knitted wires.

FIG. 7 shows another alternative construction for the electrodesegments, in the form of a woven mesh or screen 78 on an electricallyinsulative backing 80. This type of electrode segment construction isparticularly well suited for epicardial positioning, e.g. with electrode44 in FIG. 4.

Another alternative segment construction, shown in FIGS. 8 and 9,involves a flexible, electrically insulative cylindrical core 82 ofpolyurethane, medical grade silicone rubber, or other suitable bodycompatible material. Core 82 is surrounded by an electrically conductivecoil winding 84, preferably a wire or composite cable such asillustrated in FIG. 2. The helically wound coil conductor provides thegreatest flexibility and fatigue resistance of any of the arrangementsdiscussed, and for this reason is preferred in the case of directepicardial attachment, or any other implant location in which the leadsegments are subject to continued or repeated muscular contraction orother abrupt tissue movements. A disadvantage, relative to otherembodiments, is that a helical coil electrode segment, as compared toother segments of equal length, involves a substantially longerconductive path with less tensile strength.

All of the alternative constructions provide electrode segments whichare highly compliant, first in the sense that they readily adjust to thecontours of body tissue at the implant site when they are implanted, andsecondly over the long term, in continually conforming to the tissueduring muscular contractions and other tissue movement.

FIG. 10 illustrates a further embodiment defibrillation electrode 86including electrode segments 88, 90 and 92 formed as branches, radiatingor extended outwardly from a common junction and stress relief area 94.Junction 94 is positioned at the distal tip region of a lead 96 to apulse generator (not shown), and includes a conductive portionsurrounded by an insulative sheath 98. The conductive region of the leadand the electrode segments can be constructed as previously described.

The stress relief portion of the electrode is electrically insulativeand covers portions of the segments, leaving exposed portions of thesegments spaced apart from one another and defining an effective orphantom area 100 shown by the broken line. As before, segments 88-92have a nominal width preferably about 0.5 mm, and are longer than theyare wide, for example by at least a factor of five. At the free ends ofthe segments are respective masses or bodies 102, 104 and 106. Thebodies are constructed of an electrically conductive, plasticallydeformable material such as platinum or gold and, as seen in FIG. 10,include slots 108 slightly wider than the thickness of segments 88-92.Each body is applied to the free end of its respective electrode segmentby inserting the free end within the respective slot and pinching thebody to frictionally secure the body to the electrode segment. Bodies102-106 thus provide enlargements at the free ends of the segments toreduce the chance for high current densities at the free ends, andprovide a means of fixation of the free ends.

FIGS. 11-13 schematically illustrate alternative configurations forelectrode 86. More particularly, FIG. 11 illustrates a clamp 110 forelectrically and mechanically coupling two intersecting cables 112 and114. Cable 112 is part of lead 96, with a distal portion of the leadproviding center segment 90. Electrode segments 88 and 92 are oppositeportions of cable 114. An extension 116 of electrically insulativesheath 98 covers clamp 110 and portions of cables 112 and 114, leavingthe segments exposed.

In FIG. 12, segments 88, 90 and 92 extend radially from a crimpingmember 118 at the distal end of lead 96. Alternatively, segment 90 isthe distal end of the lead, in which case the remainder of the lead,crimping member 118 and portions of the electrode segments are providedwith an insulative covering 119.

In FIG. 13, crimping member 118 secures electrode segments 88, 90 and 92to the distal section 120 of lead 96. Insulative sheath 98 leaves distalsection 120 exposed, so that it functions as a fourth electrode segment.

FIG. 14 shows a further embodiment defibrillation electrode 122including a lead 124 having a distal end 126 formed in a curved,serpentine configuration. An insulative sheath 128 covers the lead andleaves the distal region exposed. Further insulation covers curvedportions of the electrode at 130, 132 and 134, thus to define fourparallel segments or length-portions 136, 138, 140 and 142 aligned withone another and side by side.

FIGS. 15, 16 and 17 disclose alternative serpentine electrodeconfigurations including an electrode 144 with a wire mesh or screen 146on an electrically insulative backing 148. FIGS. 15 and 16 illustrate aconductive path 150 including parallel electrode segments 152, 154, 156and 158. The distal end of segment 158 is enlarged at 160 to counteractedge effect current densities.

In FIG. 17, an electrode 162 includes a serpentine conductive path 164formed between a pair of generally rectangular electrically insulativelayers 166 and 167. A serpentine opening in layer 166 exposes part of awire mesh layer 168. Slits in the patches at 170, 172 and 174 allow thepatch to conform to the site of implant. Selected parts of theconductive path can be covered with insulation if desired, to leave justparallel segments exposed.

FIG. 18 discloses yet another embodiment defibrillation electrode 176 inwhich a single conductive path 178 at the distal end of a lead 180 isformed into a spiral. The path can be a coated composite cable or a wiremesh or screen as previously described, with a similar nominal width inthe radial direction. The pitch of the spiral, i.e. radial spacing (D)between adjacent arcs in the spiral, is preferably about 3 cm. Thus theeffective electrode area encompasses the outermost arc of the spiral, asindicated by the broken line at 182. The spiral includes at least twocomplete turns or length-portions as shown, with each turn forming anarcuate electrode segment to provide respective radially inward andoutward segments 184 and 186.

Regardless of the particular embodiment, electrodes constructed inaccordance with the present invention provide a substantially largereffective or phantom area than previously practical for implantabledefibrillation electrodes. One reason for this is the spacing betweenadjacent electrode segments, resulting in more compliant electrodes,both in the sense of matching contours in body tissue, and "dynamically"in responding to muscular contractions and other sudden or rapid tissuemovement, with virtually no risk of fatigue. Another feature permittingthe large size is the highly conductive electrode segments and leaddistal end or other feature electrically coupling the electrodesegments. This ensures an acceptably low voltage gradient across evenrelatively large electrodes.

As previously noted, a large but segmented electrode structure resultsin a substantially more uniform current distribution, as compared toconventional continuous patch electrodes. FIG. 19 schematicallyillustrates electrical current flow, in broken lines, between acontinuous patch electrode 187 and an electrode composed of parallel,spaced apart wires or segments 189. Adjacent segments 189 are quiteclose to one another, e.g. spaced apart from one anther a distance ofabout 5 mm. Because of the low impedance between adjacent segments 189,there is virtually no potential difference between these segments andintervening tissue. Most of the current flow is along the end segments189, and very little occurs near the intermediate segments or betweensegments. Consequently, the electrode formed of segments 189, much likeelectrode 187, exhibits a non-uniform current distribution, with veryhigh current density at the outside edges and low current density alongthe medial region.

In FIG. 20, the electrical current flow; and between two electrodes withrespective segments 191 and 193 exhibits a substantially uniform currentdensity across each electrode. Again the current flow is shown in brokenlines, and illustrates the importance of sufficient spacing betweenadjacent electrode segments. More particularly, the segments ofelectrodes 191 and 193 are spaced apart from one another a sufficientdistance for intervening tissue to provide substantial electricalimpedance between adjacent electrode segments. Thus, each of segments191 and 193, including the intermediate segments, responds to theopposite one of the electrode pair, permitting current densities, overthe central regions of these electrodes, substantially equal to thecurrent densities at their edges.

FIG. 21 shows the relationship between the spacing between coils oradjacent and parallel electrode segments, and impedance, for groups oftwo, three and four segments as shown at 195, 197 and 199, respectively.In all cases the impedance is highest when adjacent segments are closesttogether. In all cases, increasing the spacing from 1 cm to thepreferred 3 cm reduces impedance, and the cases show some furtherimprovement as spacing is increased beyond 3 cm. For any selectedspacing, the four segment electrode exhibits the lowest impedance, whichis not surprising in view of the fact that larger electrodes generallyexhibit lower impedance.

Thus, it has been found that electrode performance is substantiallyimproved, in terms of reduced impedance as well as uniformity of theelectrical field, when the spacing between adjacent segments is at least1.5 cm. The upper limit of spacing is less strict, and subject tophysical (size and patient comfort) constraints rather than electricalperformance constraints. Within these limits, the optimum spacingdepends upon the materials employed and the intended location ofimplant. Generally, however, a spacing of 3 cm between adjacentelectrode segments has been found satisfactory.

FIG. 22 schematically illustrates an implanted defibrillation systemincluding spaced apart electrodes 188 and 190, for example similar toelectrode 16. The defibrillation system further includes a pulsegenerator 192, and leads 194 and 196 connecting the pulse generator toelectrodes 188 and 190, respectively. Both of the electrodes aresubcutaneous and outside of the rib cage, in the thoracic region. Theelectrodes are on opposite sides of the heart 198. More particularly,electrode 188 is positioned to the left of, and anterior with respectto, the heart. Electrode 190 is posterior with respect to the heart, andto the right of the heart. Such transthoracic application ofdefibrillation pulses requires electrodes having a large surface area,achieved in accordance with the present invention by the spaced apartelectrode segments of each electrode. Pulse generator 192 is alsomounted anterior and to the left of heart 198, below electrode 188. Thepulse generator can incorporate circuitry for sensing cardiac electricalactivity, in which case electrodes 188 and 190 are used in sensing suchactivity as well as delivering defibrillation pulses.

FIG. 23 discloses a defibrillation system in which an electrode 200constructed in accordance with the present invention is coupled to adefibrillation pulse generator 202 by a lead 204. Another electrode 206,also constructed according to the present invention, is applied directlyto epicardial tissue. Electrode 200 is positioned inside of rib cage207, and can be within the pleural cavity if desired. Stimulation occursacross the heart, with electrode 200 to the left of the heart andelectrode 206 at the right ventricle.

FIG. 24 shows a defibrillation electrode system including an electrode208 positioned anterior of and to the left of the heart 210, as in FIG.22. A second electrode 212 is provided as a coil, near the distal end ofan intravascular catheter 214 in the right atrium and terminating at theapex of the right ventricle.

Regardless of the location of implant, electrodes, constructed inaccordance with the present invention provide relatively large (in therange of 100-300 square cm) effective areas, yet readily conform tocontours and contractions or other movement of body tissue. The narrowelectrode segments are provided with end loops or other enlargements tocounteract high current densities due to edge effects and to providefixation. The present lead configurations further allow a subcutaneousimplantation outside of the rib cage, with effective defibrillationenergy production due to large virtual sizes based on the phantom areasincorporating the electrode segments.

What is claimed is:
 1. A body implantable tissue stimulating electrodeassembly, including:an elongate, electrically conductive lead having aproximal end region and a distal end region; and an electrode includinga plurality of compliant, electrically conductive electrode segments,each of said segments having a nominal width and a length exceeding thenominal width, said electrode segments having respective and oppositefirst and second ends and being coupled to the distal end region of thelead for substantially simultaneous reception of tissue stimulatingelectrical pulses from a pulse generating means at the proximal endregion of the lead, said electrode segments being arranged in spacedapart and side-by-side relation such that each of the electrodesegments, over most of its length, is spaced apart from each one of theother electrode segments by a distance of at least 1.5 cm, each of theelectrode segments being free of electrically insulative material at andalong its periphery substantially over its entire length to provide acontinuous exposed reactive surface over substantially the entire lengthand periphery of the electrode segment, said electrode segments whenreceiving the tissue stimulating pulses cooperating to define aneffective electrode area incorporating all of the electrode segments;electrically conductive means at the respective second ends of theelectrode segments for reducing current density.
 2. The assembly ofclaim 1 wherein:each of the electrode segments is uniform in sections astaken perpendicular to said length.
 3. The assembly of claim 1wherein:said means at the respective second ends include one of thefollowing:respective electrically conductive loops, respectiveelectrically conductive tab enlargements, and respective electricallyconductive bodies, each body being secured to its respective one of thesecond ends.